A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective NA of the system and also increasing the depth of focus). Other immersion liquids have been proposed, including water with solid particles (e.g. quartz) suspended therein.
However, submersing the substrate or substrate and substrate table in a bath of liquid (see, for example, U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by reference) means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and turbulence in the liquid may lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). One way which has been proposed to arrange for this is disclosed in PCT patent application publication WO 99/49504, hereby incorporated in its entirety by reference. As illustrated in FIGS. 2 and 3, liquid is supplied by at least one inlet IN onto the substrate, preferably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. FIG. 2 shows the arrangement schematically in which liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. In the illustration of FIG. 2 the liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible, one example is illustrated in FIG. 3 in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.
In European patent application publication no. EP 1420300 and U.S. patent application publication no. US 2004-0136494, each hereby incorporated in their entirety by reference, the idea of a twin or dual stage immersion lithography apparatus is disclosed. Such an apparatus is provided with two tables for supporting a substrate. Leveling measurements are carried out with a table at a first position, without immersion liquid, and exposure is carried out with a table at a second position, where immersion liquid is present. Alternatively, the apparatus has only one table.
A problem with having a liquid in an immersion system as described above is that there is the risk of evaporation of the immersion liquid. The evaporation of the immersion liquid may have one or more consequences. A first is a possible problem of localized cooling of the substrate, resulting in the shrinkage of the substrate and thus overlay and focus errors from bimetallic bending caused by differences in the coefficient of thermal expansion between the substrate and the substrate table. Even if the substrate and the substrate table have the same coefficient of thermal expansion, a temperature gradient in the z-plane may cause bending and thereby possibly introduce focus errors. A second is the possible presence of drying stains on the substrate surface. A third consequence is the possible cooling of the liquid confinement system, which indirectly cools the projection system and may cause spherical aberrations in the projected beam. Furthermore, a temperature change caused by evaporation may cause the substrate table or the substrate holder sizes and shapes to change. The position stability and signals of the sensors on the substrate table may be prone to drifting if, for example, the substrate table changes shape. Finally, indirect cooling of the immersion liquid by the cooled liquid confinement system may result in refractive index variation in the liquid, which in turn may cause focus offsets and distortions in the projected beam and thus in the pattern.
The operation of the lithographic apparatus as a whole is state dependent and so any variation in energy in any of the parts of the system may lead to errors in the exposure of the resist layer on the substrate. The concentration difference in relative humidity just above a liquid film on the substrate, substrate table, substrate holder and sensors may cause the liquid in the liquid film to evaporate, causing one or more (or other) of the problems outlined above.
There are several places in the immersion system where evaporation of the immersion liquid may occur. These are:
1. between the liquid confinement system and the substrate;
2. on the surface of the substrate after wetting by the liquid confinement system;
3. in the exhaust channels inside the liquid confinement system; and/or
4. in a gutter or gap between the substrate holder and the substrate table containing the substrate holder.
Evaporation at these places may lead to temperature reduction and therefore may lead to energy loss in or at the projection system, the immersion fluid, the liquid confinement system, the sensors, the substrate, the substrate holder and the substrate table that contains the substrate holder and the substrate. The substrate table is sometimes referred to as a “mirror block” and the substrate holder is sometimes referred to as a “pimple plate” because of the pimples that often cover it surface to support the substrate. The substrate table supports the substrate holder, which in turn holds the substrate away from the surface of the substrate table.
There are one or more reasons why net evaporation would be greater than net condensation, possibly causing a net loss of energy in the system. A first possible reason is the use of a gas knife, which is used to push the liquid film on the substrate back and drag it forward as the substrate moves in and out of the immersion system. The gas knife does this using a pressurized gas jet. Evaporation is a possible side effect of the movement of the liquid film. If liquid loss from the surface of the substrate is too large, the liquid film pushed by the gas knife is too large and the gas knife collapses. If this occurs, the liquid film evaporates outside the immersion system in the ambient environment.
A second possible reason is when the gas knife is not used or not completely effective, the substrate and substrate table are moved relative to the liquid confinement structure, so that a portion of the substrate and substrate table will be exposed to the atmosphere surrounding the lithographic apparatus. Any liquid remaining on the substrate or substrate table is likely to evaporate into the atmosphere, thus reducing the energy of the system. Evaporation of liquid from the liquid supply or exhaust system cools down the entire liquid confinement system, which cools down the immersion liquid, thus reducing the energy of the system and causing one or more (or other) of the problems described above.